1. Field of the Invention
The present invention relates to an arc tube for a discharge lamp device, in which the arc tube comprises a pair of tungsten electrode rods that are sealingly attached to pinch seal portions disposed in both ends of a glass tube, respectively, and that are opposed to each other in a closed glass bulb disposed in a middle of the glass tube with projecting tip end portions of the electrode rods thereinto, a starting rare gas, a light emitting substance, and the like being enclosed in the glass tube. More particularly, the present invention relates to an arc tube in which mercury that is an environmental toxic substance is not enclosed in the closed glass bulb (hereinafter, such an arc tube is referred to as a mercury-free arc tube).
2. Related Art
FIG. 9 shows a conventional discharge lamp device. The discharge lamp device has a structure in which a front end portion of an arc tube 5 is supported by one lead support 2 that is projected in front of an insulating base 1, a rear end portion of the arc tube 5 is supported by a recess 1a of the base 1, and a rear end side of the arc tube 5 is grasped by a metal supporting member S fixed to the front face of the insulating base 1.
A lead wire 8 on the front end side which is drawn out from the arc tube 5 is fixed to the lead support 2 by welding. A lead wire 8 on the rear end side penetrates a bottom face wall 1b formed in the recess 1a of the base 1, and is fixed to a terminal 3 disposed in the bottom face wall 1b by welding. The reference letter G denotes a cylindrical UV-blocking globe which cuts off UV components of the wavelength range that is harmful to the human body, from the light emitted from the arc tube 5, and which is integrated with the arc tube 5 by welding.
The arc tube 5 is structured so that a closed chamber portion (closed glass bulb) 5a is formed in which electrode rods 6, 6 are opposingly disposed between a pair of front and rear pinch seal portions 5b (a primary pinch seal portion 5b1, a secondary pinch seal portion 5b2), and a rare gas, mercury, a light emitting substance, and the like are enclosed. Molybdenum foils 7 through which the electrode rods 6 projected into the closed chamber portion 5a are connected to the lead wires 8 drawn out from the pinch seal portions 5b are sealingly attached into the pinch seal portions 5b, respectively, thereby ensuring the gas-tightness of the pinch seal portions 5b. 
Most preferably, the electrode rods 6 are made of tungsten having superior durability. However, tungsten is largely different in coefficient of linear expansion from quartz glass which constitutes the arc tube, poor in compatibility with quartz glass, and has inferior gas-tightness. Therefore, the molybdenum foils 7 which are relatively excellent in compatibility with glass are connected to the tungsten electrode rods 6, and sealingly attached by the pinch seal portions 5b, whereby the gas-tightness of the pinch seal portions 5b is ensured. The electrode rods 6, the molybdenum foils 7, and the lead wires 8 are previously integrated with one another as electrode assemblies A1, A1′.
FIG. 10 shows an arc tube disclosed in JP-A-2001-015067. In the periphery of the electrode rod 6 in each of the pinch seal portions 5b (5b1, 5b2), a residual compression strain layer 9 surrounding the electrode rod 6, and an interface crack 9a elongating along the interface between the residual compression strain layer 9 and an outside glass layer are formed so as to suppress generation of a vertical crack (a crack which elongates from the periphery of the electrode rod toward the surface of the pinch seal portion) which may cause lighting failure or the like, in the pinch seal portion 5b. When the arc tube lights ON, specifically, thermal stress is generated between the electrode rod 6 and the glass layer in which the temperature difference between light-ON/OFF periods of the arc tube is large, and which are largely different in coefficient of linear expansion from each other. Particularly, recent arc tubes are configured so as to enable instant lighting, and have a large rate of temperature rise. In such an arc tube, therefore, thermal stress is rapidly generated. When this state is repeated, a vertical crack is generated at a sealingly attached position of the pinch seal portion (glass layer) 5b to which the electrode rod 6 is sealingly attached, and the substances enclosed in the closed chamber portion 5a leaks, thereby causing a possibility that lighting failure or reduction of the life period may occur. However, the residual compression strain layer 9 surrounding the electrode rod 6, and the interface crack 9a efficiently relax (absorb) thermal stress which is generated in the glass layer in accordance with temperature rise. In this structure, therefore, a vertical crack which may cause the enclosed substances to leak is not generated in the pinch seal portion (glass layer) 5b. 
In the closed glass bulb 5a of the conventional arc tube of these type, mercury which performs a buffer function (a function of maintaining an adequate tube voltage) is enclosed. However, mercury is an environmental toxic substance. In order to satisfy social needs that environmental contamination of the earth is reduced as far as possible, there is a mercury-free arc tube, as a related art of the present invention, in which mercury is not enclosed in a closed glass bulb.
However, in the related art mercury-free arc tube, a novel problem that a vertical crack, which is hardly generated in a mercury arc tube, is generated in a pinch seal portion has arisen.
The problem is caused by the following phenomenon. In the pinch seal portion of the mercury-free arc tube, the residual compression strain layer (interface crack) around the electrode rod is formed larger (the radius of the residual compression strain layer or the interface crack is larger) than that in the mercury arc tube. Therefore, the glass layer outside the residual compression strain layer (interface crack) in the pinch seal portion is correspondingly made thinner. When thermal stress which exceeds the stress absorption limit of the residual compression strain layer (interface crack) (hereinafter, referred to as excessive thermal stress) acts on the pinch seal portion, a vertical crack is generated.
That is, as a countermeasure against reduction of the tube voltage due to mercury free, a mercury-free arc tube is configured so as to thicken an electrode rod in order to increase the tube current, thereby maintain the tube power. However, the configuration of a pincher for shaping the pinch seal portion (the sizes of a section face of the pinch seal portion, and the like) is identical with that in the case of a mercury arc tube. By contrast, in the pinch seal portion, the amount of thermal contraction of the electrode rod after pinch seal is larger correspondingly with the thickness of the electrode rod, and also the residual compression strain layer (interface crack) formed around the electrode rod is larger (the radius of the residual compression strain layer or the interface crack is larger) than that in the case of a mercury arc tube. Consequently, the glass layer outside the residual compression strain layer (interface crack) in the pinch seal portion is correspondingly thinner than that in a mercury arc tube. When excessive thermal stress acts on the pinch seal portion, therefore, a vertical crack is generated.